Grass mowing machines known as zero turning radius (“ZTR”) mowers have at least one independently powered rear drive wheel on each side of a frame. Many ZTR mowers have a twin stick control system. A pair of motion control levers or sticks may be provided side-by-side, with each lever or stick controlling one of the rear drive wheels. When both levers or sticks are advanced together forwardly out of their neutral position, both rear drive wheels rotate forwardly to cause the ZTR mower to move forward. A ZTR mower may be steered by advancing one lever or stick more than the other.
Typically, each motion control lever or stick on a ZTR mower maybe linked to a pump arm of one of two separate hydraulic pumps, or of a dual hydraulic pump; i.e., a separate pump for each rear drive wheel. The lever or stick may be used to move a pump swash plate through a direct linkage.
Most ZTR mowers rely on the rear drive wheels for propulsion, steering and braking, while the front wheels are on casters. When a ZTR mower is traversing a slope, the front wheels provide no holding force to the front of the ZTR mower to keep it moving straight across the slope. Instead, to keep the ZTR mower moving straight across the slope, the operator may pull back the motion control lever or stick for the uphill rear drive wheel, so that it acts as a brake and the wheel slides against the ground. As a result, the downhill rear drive wheel may be the only wheel that provides traction. If the downhill rear drive wheel slips, the mower may slide down the hill.
A zero turn mower steering system is needed having front wheels that provide a holding force to the front of the mower to keep it moving straight across a slope and that can enable the mower to maneuver around obstacles located on slopes.